a couple qestions.

I agree with what they said, fabric will look to out of scale unless it is a very large scale.

You do not mention scale.  A sewn sandbag could be very suitable at 1/6 scale; less so at 1/16.  By 1/32 or 1/35 the Milliput/A+B putty method is best.

Size will approximate that of a 40# flour or rice sack.  Call it 16-18" x 24" x  4-6-8" thick.

Note that sandbags are often no more than 2/3 full.

Also note that fabric bags have given away to spun fiber, and, lately, coated films.  Both of those latter two are made from tubes of the material used, with one closed end..  This gives then a unique look in practice, as the unfilled end is rolled, or tucked under the rest

What he said and true clay works pretty well too. The dirt colored kind.

Roll out a long piece say 1/4" diameter, like Gnocchi.

Cut into lengths and squash the ends.

The fabric will look too out of scale.  The best material for making sand bags is A & B epoxy putty.  It is two-parts of different colors and looks like modeling clay.  Mix the two parts together until you have a uniform color and form into about 3/4 inch long by 1/2 inch wide and 1/8 inch thick rectangles to make the sand bags.  Scract a line into each short end to represent the seam on them.  Gently press the sandbags onto a cotton t-shirt to give them some texture.  Form them over whatever they are on to take that shape and let them cure for a few hours.  When done, paint and weather and you will have great lookiing sandbags.   Also, before they harden, you can use an xacto knife to put some tears and holes in them to make them look worn  and used.  Believe me, they will look much better.

Here is a tutorial that explains it as well.

armorama.kitmaker.net/modules.php